// JavaScript Document
 var $ = function(o){return document.getElementById(o);}
  window.onload = function(){
   window.onscroll = function(){
    var scrollTop = document.documentElement.scrollTop;
    if(scrollTop){
     $("backtop").style.display = 'block';
     $("backtop").style.top = (scrollTop + document.documentElement.clientHeight - 80) + "px";
    }else{
     $("backtop").style.display = 'none';
    }
   }
  }